I need tactics to catch my boyfriend cheating? Ideas welcome!!

My boyfriend is friends with this girl, who I know from many reliable sources that they hang out ALONE plenty. I would actually be okay with this, except he has never mentioned her to me. I have actually seen them together too. I don't trust my boyfriend with her because she is very flirty and they have attraction between each other. She also dresses very provocatively. And there's nothing wrong with it, I'm sure she is a very nice girl, but nice people can do wrong things. My boyfriend has also cheated on some of his past girlfriends, although he tells me I'm different than them. I just need some good ideas to catch them in the act so I have concrete proof? I really love my boyfriend and I hope I'm wrong, but I also don't want to be with him if he isn't happy in our relationship. Thanks!

I agree with Phoenix1991. You're talking about tactics. Sure he is in the wrong if he's cheating on you but you do sound manipulative. If he cheated on his ex, he will cheat on you. Don't think he'll be different for you. You don't trust him anymore and it will always be that way. Each time he goes out, you'll wonder what he's doing. You'll be overly jealous and it will ruin the relationship, it won't be healthy.